Gas-filled detectors are arranged in three various forms, those forms being ion chambers, proportional counters, and Geiger-Mueller counters. Of these basic forms, the proportional counter design is often used as the fundamental instrument for a gas-filled neutron detector. Proportional counters rely upon avalanche multiplication in the gas to produce large electronic signals, each signal being proportional to the energy deposited in the detector chamber. Quite differently, ion chambers do not produce avalanche multiplication and Geiger-Mueller counters produce excessive avalanching such that the electronic signal is no longer proportional to the energy deposited in the chamber.
Gas-filled proportional counters used for neutron detectors can be further subdivided into two types, those being detectors filled with a neutron reactive gas and those detectors that are coated with a neutron reactive material. Neutron interactions in either the neutron reactive gas or the neutron reactive coating eject energetic charged particles that create ionization in the detector gas. A voltage applied to the gas chamber causes the ions and electrons to move, and this ionization is subsequently measured as a current thereby indicating a neutron interaction occurred.
The most popular type of gas-filled neutron detector is the type filled with a neutron reactive gas. However, in recent times, these neutron reactive gases have been deemed hazardous, as is the case for BF3, or have become rare and difficult to acquire, as is the case for 3He. The gas-filled neutron detector design with neutron reactive materials coating the walls does not suffer from these issues, yet these coated detectors have an intrinsic problem in that they are limited to relatively low neutron detection efficiency. The low detection efficiency is a direct result of the reaction products having a limited range in the coating, hence any coating thicker than the reaction product particle ranges simply absorbs all of the particle energy, which is therefore not transferred to the detecting gas.
The converter films attached to gas-filled proportional counters most often used for neutron detection utilize either the 6Li(n,t)4He reaction or the 10B(n,α)7Li reactions. Due to low chemical reactivity, the most common materials used are pure 10B and 6LiF. Neutron reactive films based on the 157Gd(n,γ)158Gd reaction show a higher neutron absorption efficiency than 10B(n,α)7Li and 6Li(n,α)3H-based films, however the combined emission of low energy gamma rays and conversion electrons from 157Gd(n,γ)158Gd reactions make neutron-induced events difficult to discriminate from background gamma-ray events. As a result, Gd-based films are less attractive for devices where background gamma ray contamination is a problem. Alternatively, the particle energies emitted from the 6Li(n,t)4He and the 10B(n,α)7Li reactions are relatively large and produce signals easily discernable from background gamma ray noise. Thus far, thermal neutron detection efficiencies have been limited to only 4% for 6LiF and 10B single-coated devices. However, devices that utilize pure 6Li as the converter can have efficiencies as high as 13% for a single coated device. Unfortunately, pure Li decomposes rapidly in most circumstances, making a pure Li coated device impractical at present. As a result the most commonly used neutron converter films are B and LiF, both of which are poor electrical conductors. There are some cases in which fissionable material, such as 235U, 238U and 232Th are used in gas-filled detectors, yet these same coatings are generally used for gas-filled ion chambers.
The 10B(n,α)7Li reaction leads to the following reaction products:
                             10            ⁢      B        +                           0        1            ⁢      n        ->      {                                                                                                                                                         7                                        ⁢                    Li                                    ⁡                                      (                                          at                      ⁢                                                                                          ⁢                      1.015                      ⁢                                                                                          ⁢                      MeV                                        )                                                  +                                  α                  ⁡                                      (                                          at                      ⁢                                                                                          ⁢                      1.777                      ⁢                                                                                          ⁢                      MeV                                        )                                                              ,                                                                                                                                                                             7                                        ⁢                    Li                                    *                                      (                                          at                      ⁢                                                                                          ⁢                      0.840                      ⁢                                                                                          ⁢                      MeV                                        )                                                  +                                  α                  ⁡                                      (                                          at                      ⁢                                                                                          ⁢                      1.470                      ⁢                                                                                          ⁢                      MeV                                        )                                                              ,                                          ⁢                                                                  2.792                ⁢                                                                  ⁢                MeV                ⁢                                                                  ⁢                                  (                                      to                    ⁢                                                                                  ⁢                    ground                    ⁢                                                                                  ⁢                    state                                    )                                                                                                        2.310                ⁢                                                                  ⁢                MeV                ⁢                                                                  ⁢                                  (                                      1                    ⁢                    st                    ⁢                                                                                  ⁢                    excited                    ⁢                                                                                  ⁢                    state                                    )                                                                                          Reaction            ⁢                                                  ⁢            Q            ⁢                          -                        ⁢            Value                    _                    which are released in opposite directions when thermal neutrons (0.0259 eV) are absorbed by 10B. After absorption, 94% of the reactions leave the 7Li ion in its first excited state, which rapidly de-excites to the ground state (˜10−13 seconds) by releasing a 480 keV gamma ray. The remaining 6% of the reactions result in the 7Li ion dropping directly to its ground state. The microscopic thermal neutron absorption cross-section is 3840 barns. Additionally, the microscopic thermal neutron absorption cross-section decreases with increasing neutron energy, with a dependence proportional to the inverse of the neutron velocity (1/ν) over much of the energy range.
The 6Li(n,t)4He reaction leads to the following products:
                                         6                ⁢        Li            +                                   0          1                ⁢        n              ->                                                     3                    ⁢          H                ⁡                  (                      at            ⁢                                                  ⁢            2.73            ⁢                                                  ⁢            MeV                    )                    +              α        ⁡                  (                      at            ⁢                                                  ⁢            2.05            ⁢                                                  ⁢            MeV                    )                      ,            4.78      ⁢                          ⁢      MeV                      Reaction        ⁢                                  ⁢        Q        ⁢                  -                ⁢        Value            _      which again are oppositely directed if the neutron energy is sufficiently small. The microscopic thermal neutron (0.0259 eV) absorption cross-section is 940 barns. The thermal neutron absorption cross-section also demonstrates a 1/ν dependence, except at a salient resonance above 100 keV, in which the absorption cross-section surpasses that of 10B for energies between approximately 150 keV to 300 keV. Additional resonances characteristic to either isotope cause the absorption cross-section to surpass one or the other as the neutron energy increases. Due to its higher absorption cross-section, the 10B(n,α)7Li reaction leads to a generally higher reaction probability than the 6Li(n,t)4He reaction for neutron energies below 100 keV. However, the higher energy reaction products emitted from the 6Li(n,t)4He reaction lead to greater ease of detection than the particles emitted from the 10B(n,α)7Li reaction.
The term “effective range” (denoted L) is the distance through which a particle may travel within the neutron reactive film before its energy decreases below the set minimum detectable threshold, or rather, before its energy decreases below the electronic lower level discriminator (LLD) setting. The term does not take into account additional energy losses from contact “dead regions”. The neutron reaction products released do not have equal masses, and therefore do not have equal energies or effective ranges. Neutrons may interact anywhere within the reactive film, and the reaction products lose energy as they move through the neutron reactive film. Reaction product self-absorption reduces the energy transferred to the detector gas, and ultimately limits the maximum film thickness that can be deposited over the detector. The measured voltage signal is directly proportional to the number of ion pairs excited within the detector gas. Reaction products that deposit most or all of their energy in the detector will produce much larger voltage signals than those reaction products that lose most of their energy before reaching the detector.
The energy absorbed in the detector is simply the original particle energy minus the combined energy lost in the reactive absorber film and the detector contact during transit. At any reaction location within the reactive film, a reduced energy will be retained by either particle that should enter the detector, being the maximum possible if the trajectory is orthogonal to the device contact. Hence, if the interaction occurs in the 10B film at a distance of 0.5 μm away from the detector gas, the maximum energy retained by the 7Li ion when it enters the detector gas will be 430 keV, and the maximum energy retained by the alpha particle will be 1150 keV. For the same interaction distance of 0.5 μm from the detector, the energy retained by the particle when it reaches the detector gas decreases as the angle increases from orthogonal (>0°). Given a predetermined minimum detection threshold (or LLD setting), the effective range (L) for either particle can be determined For instance, an LLD setting of 300 keV yields LLi as 0.810 microns and Lα as 2.648 microns. Similar conditions exist for 6LiF and 6Li films.
A commonly used geometry involves the use of a cylindrical gas-filled detector over which a neutron reactive film has been deposited inside the cylinder. Assuming that the neutron beam is perpendicular to the cylinder wall and reactive film, the sensitivity contribution for a reaction product species can be found by integrating the product of the neutron interaction probability and the fractional solid angle, defined by the reaction product effective ranges subtending the device interface, which yields:
                                                        S              p                        ⁡                          (                              D                F                            )                                =                                    0.5              ⁢                              F                p                            ⁢                              {                                                                            (                                              1                        +                                                  1                                                                                    Σ                              F                                                        ⁢                            L                                                                                              )                                        ⁢                                          (                                              1                        -                                                  ⅇ                                                                                    -                                                              Σ                                F                                                                                      ⁢                                                          D                              F                                                                                                                          )                                                        -                                                            D                      F                                        L                                                  }                            ⁢                                                          ⁢              for              ⁢                                                          ⁢              D                        ≤            L                          ,                                  ⁢        and                            (                  1          ⁢          A                )                                                                    S              p                        ⁡                          (                              D                F                            )                                =                                    0.5              ⁢                                                          ⁢                              F                p                            ⁢                              ⅇ                                  -                                                            Σ                      F                                        ⁡                                          (                                                                        D                          F                                                -                        L                                            )                                                                                  ⁢                              {                                                                            (                                              1                        +                                                  1                                                                                    Σ                              F                                                        ⁢                            L                                                                                              )                                        ⁢                                          (                                              1                        -                                                  ⅇ                                                                                    -                                                              Σ                                F                                                                                      ⁢                            L                                                                                              )                                                        -                  1                                }                            ⁢                                                          ⁢              for              ⁢                                                          ⁢                              D                F                                      >            L                          ,                            (                  1          ⁢          B                )            where ΣF is the macroscopic neutron absorption cross-section, DF is the film thickness, and Fp is the branching ratio of the reaction product emissions. The total sensitivity accordingly can be found by adding all of the reaction product sensitivities
                                                        S              ⁡                              (                                  D                  F                                )                                      ⁢                          ❘              Total                                =                                    ∑                              p                =                1                            N                        ⁢                                          S                p                            ⁡                              (                                  D                  F                                )                                                    ,                            (        2        )            where N is the number of different reaction product emissions. In the case of 10B-based films, N equals 4. Notice from equation 1B that the value of Sp reduces as DF becomes larger than the value of L. As a result of this, there will be an optimum neutron reactive film thickness for front-irradiated detectors. Because the minimum particle detection threshold determines the effective range (L), the optimum film thickness is also a function of the LLD setting. With the LLD set at 300 keV, the maximum achievable thermal neutron detection efficiency is 3.95%. The thermal neutron detection efficiency can be increased to 4.8% by lowering the LLD setting, but only at the expense of accepting more system noise and gamma-ray background interference. Similar cases exist for 6LiF and pure 6Li films. Using an LLD setting of 300 keV, obverse detector irradiation yields maximum thermal neutron detection efficiencies of 4.3% for 6LiF-coated devices and 11.6% for pure 6Li-coated devices. Hence, neutrons entering and exiting the gas detector are limited to a total efficiency of approximately 8.3%.Design of Conventional 10B and 6Li Coated Gas-Filled Proportional Detectors
In 1908, Ernest Rutherford and Hans Geiger constructed a device composed of a metallic cylinder with a thin wire arranged axially inside. The gas medium in the device was simply air. With the application of a voltage, alpha particles projected into the device produced sizable currents as measured with an electrometer. Rutherford and Geiger had devised the first radiation counter. They also noticed that the behavior of the detector changed with increasing voltage, mainly that alpha particles could be detected at much lower applied voltages than beta particles, a technique and application that later became known as proportional counting. Experiments conducted with the gas-filled detectors clearly showed distinctive regions of operation, as shown in FIG. 1.
The principle behind a gas-filled detector is quite simple. Radiation interactions in the gas or ejected particles from radiation interactions in the chamber walls cause the detector gas to become ionized, and a charge cloud composed of electrons and positive ions appears. A voltage placed across electrodes in the gas chamber causes the electrons and ions to drift apart, where electrons drift towards the anode and the positive ions drift towards the cathode. As the charged particles, or charge carriers, move through the chamber, they induce current to flow in a circuit externally connected to the chamber. This current, or change in current, can then be measured as an indication that a radiation interaction occurred in the chamber.
General Operation
Gas detectors can be operated in pulse mode or current mode. Pulse mode is generally used in low to moderate radiation fields. In such a case, a single radiation quantum, such as an alpha particle, beta particle or gamma ray, interacts in the chamber volume, giving rise to an ionized cloud. The charge carriers drift apart, and as they move, they induce current to flow to the device terminals; a charging circuit, usually consisting of a preamplifier and feedback loop, integrates the current and stores the charge, thereby producing a voltage potential. This voltage is measured as a single event, indicating that a single radiation quantum has been detected. The preamplifier circuit is subsequently discharged and reset, allowing the device to measure the next radiation interaction event. Hence, each voltage pulse from the detector indicates an individual radiation interaction event. Although extremely useful, there are drawbacks to this method. Should another radiation interaction occur while the detector is integrating or discharging the current from a previous interaction event, the device may not, and usually does not, record the new interaction, a condition referred to as pulse pile up. The time duration in which a new pulse can not be recorded is the detector recovery time, sometimes referred to as dead time. A pulse mode detector operated in low radiation fields has little problem with dead time count losses; however, a detector operated in high radiation fields may have significant dead time losses, thereby yielding an incorrect measurement of the radiation activity in the vicinity.
For high radiation fields, gas detectors are operated in current mode, in which the radiation induced current is measured on a current meter. Under such conditions, many interactions can occur in the device in short periods of time, and the current observed increases with total radiation exposure rate. Hence, current mode can be used to measure high radiation fields, with the magnitude of the current being a measure of the radiation induced ionization rate in the detector, thereby giving a measure of the radiation field in which the device is being operated. The disadvantage of current mode is that it does not identify individual radiation interactions.
FIG. 2 illustrates a gas-filled detector similar to that first explored by Geiger and Rutherford. The detector is exposed to directly ionizing radiation, which would include α-particles and β-particles. Either of these particles can cause ionization in the gas-filled device, thereby, producing electron-ion pairs. Hence, there are both an absorber and an observable, so that to produce a radiation detector only a method is needed to measure the amount of ionization. Suppose the device is connected to a simple electrometer so as to measure the current produced by the motion of the electron-ion pairs. Without an applied voltage, the electron-ion pairs diffuse randomly in all directions and eventually recombine. As a result, the net current from the electrometer is zero. Now apply a positive voltage to the thin wire of the device, or anode, so that the free electrons (negative charge) drift towards the anode and the free ions (positive charge) drift towards the detector wall. At low voltages, some measurable current is seen, yet considerable recombination still occurs, which is the recombination region identified as Region I in FIG. 1. As the voltage is increased, electron-ion pair separation becomes more efficient until practically no recombination occurs. Hence, the current measured is a measure of the total number of electron-ion pairs formed, which is Region II of FIG. 1, and is referred to as the ionization chamber region.
As the voltage is increased further, the electrons gain enough kinetic energy to create more electron-ion pairs through impact ionization. This provides a mechanism for signal gain, often referred to as gas multiplication. As a result, the observed current increases as the voltage increases, but is still proportional to the energy of the original radiation particle. This multiplication occurs in Region IIIa, the proportional region. Increasing the applied voltage further causes disproportional current increases to form, marked in FIG. 1 as Region Mb, beyond which, in Region IV, all currents, regardless of origin, radiation species or energies, are the same magnitude. Region IV is the Geiger-Mueller region. Finally, excessive voltage drives the detector into Region V where the voltage causes sporadic arcing and other spontaneous electron emissions to occur, hence causing continuous discharging in the detector. Gas detectors should not be operated in the continuous discharge region. In the following subsections, detector operation in Regions II, III, and IV is described in more detail.
Operation of Ion Chambers
The simplest gas-filled detector is the ion chamber. There are many configurations of ion chambers, and they are operated in Region II of the gas curve shown in FIG. 1. The detection method is simple. Ionizing radiation, such as alpha or beta particles, or gamma or x rays, enter into a region filled with a gas such as Ar or air. The chamber has electrodes across which a voltage is applied. When radiation interactions occur in the gas, they cause the gas to become ionized, which produces electron-ion pairs relative in number to the radiation energy absorbed. The voltage applied across the electrodes causes the negative electrons to separate from the positive ions and drift across the chamber volume. Electrons drift towards the anode and positive ions drift towards the cathode, and their movement induces current to flow in the external circuit. Typically, this induced current is sensed by either directly measuring the current or by storing the charge in a capacitor and measuring the resulting voltage.
The first case is referred to as current mode operation and the second case is pulse mode operation. Current mode operation is used in high radiation fields, and the magnitude of the current measured gives a measure of the intensity of the radiation field. Pulse mode is used for lower radiation fields, and allows for each individual radiation interaction in the chamber to be counted. Ion chambers come in many forms, and can be used for reactor power measurements, where the radiation field is very high, or as small personnel dosimeters, for use where radiation levels are typically low. Although simple in concept, two main problems occur in the ion chamber for pulse mode operation, those being (1) the signal measured is small, due to the fact that the current measured is only from the primary (or initial) electron-ion pairs excited by the radiation quantum and (2) the signal formation time can be long due to the slow motion of the heavy positive ions. Often, an RC circuit is connected to an ion chamber to reduce the time constant of the system and discharge the capacitor before all of the ions are collected, thereby reducing the time response.
Operation of Neutron-Sensitive Ion Chambers
If an ion chamber is coated with a strongly-absorbing neutron-reactive material or filled with a neutron reactive gas, such that ionizing particles are released from the neutron reactions, it can be used as a neutron detector. Commonly used isotopes for neutron detectors are 3He, 10B, 6Li, and 235U. Neutron sensitive ion chambers are usually filled with 10BF3 or 3He gas, or the inside walls of the chamber are coated with 10B, 6LiF, or 235U. These gas-filled neutron detectors can be operated as ion chambers or proportional counters.
Ion chambers that use 235U are often referred to as fission chambers, since it is the fission fragments from the 235U that ionize the chamber gas. Fission chambers are often used where there is a mixed radiation field containing a large component of gamma rays. Fission fragments can deposit as much as 50 times the energy as gamma rays in a fission chamber. Hence, when operated in pulse mode, the voltage pulses formed by fission fragments are much larger than gamma-ray pulses, thereby, making it possible to discriminate between the two radiations. Due to problems with pulse pile up, ion chambers and fission chambers are generally not operated in pulse mode when in high radiation fields, although some special pulse mode designs incorporating 235U are used for in-core nuclear reactor monitoring.
Proportional Counters
Observe in FIG. 1 that Region III is separated into subregions, namely, Region IIIa (proportional) and Region IIIb (limited proportionality). Proportional counters are operated in region Ma of the gas curve, in which an electronic pulse produced by ions moving through the detector is proportional to the original energy absorbed in the detector by a quantum of radiation, be they charged particles, neutrons, gamma rays or x rays. Although the gas-flow proportional counter was invented in 1943 by John Simpson, the actual effect of pulse height proportionality was known from those initial experiments conducted by Rutherford and Geiger with their gas-filled chambers. Ar is the most commonly used gas in a proportional counter, although there are many other gases that can be used, which include 3He, Xe, and 10BF3.
Let us understand just exactly how the proportional counter operates. As with the ion chamber, a quantum of radiation can interact in the device's volume, either with the gas or with the chamber walls. If, for instance, a gamma ray interacts with the chamber wall, an energetic electron can be ejected into the gas volume, which then produces a cloud of electron-ion pairs. If the gamma ray interacts directly with the gas, then the primary energetic electron again produces a cloud of electron-ion pairs. In either case, a cloud of electron-ion pairs is formed in which the total number of ion pairs produced is proportional to the radiation energy deposited in the detector. Hence, by measuring the number of ion pairs formed, the energy deposited in the gas volume by the interacting radiation quantum can be determined This measurement can be performed by applying a voltage across the detector and measuring the current produced as the electrons and ions drift through the chamber volume. Yet, as explained with the ion chamber, such a current can be minuscule and hard to measure.
At high enough voltages, electrons can gain enough kinetic energy to cause more ionization and excitation in the gas, an effect called impact ionization. These newly liberated electrons gain enough energy from the electric field to cause even more ionization. The process continues until the electrons are collected at the anode. The entire process of generating the impact ionization cloud is called a Townsend avalanche, or sometimes gas multiplication, as illustrated in FIG. 3. There is a critical electric field EA at which gas multiplication begins and below which the electrons do not gain sufficient energy to cause impact ionization. This threshold electric field defines the difference between Region II and Region III in the gas curve.
Parallel plate detector configurations may work for ion chambers, but are seldom used for proportional counters. A preferred geometry is a coaxial configuration, as depicted in FIGS. 2 and 4. To understand why, compare the difference in electric fields between coaxial and parallel plate geometries, as shown below.
Consider the parallel plate detector configuration shown in FIG. 4. If the voltage is Vo at x=x1 and zero (grounded) at x=x2, then it can be shown that the electric field is
                                          E            ⁡                          (              x              )                                =                                                    V                O                                                              x                  2                                -                                  x                  1                                                      =                                          V                O                            W                                      ,                            (        3        )            where W is the width between the parallel contacts. Notice that the electric field for the planar configuration is constant, hence a relatively large voltage is required to reach the critical avalanching field EA.
Now consider the coaxial case also shown in FIG. 4. It can be shown that, for a voltage Vo applied to the inner anode with the outer surface at ground potential, the electric field at radial distance r is
                                          E            ⁡                          (              r              )                                =                                    V              O                                      r              ⁢                                                          ⁢                              ln                ⁡                                  (                                      a                    /                    b                                    )                                                                    ,                            (        4        )            where a is the radius of the inner anode and b is the radius of the cathode shell wall. Unlike the planar case, the electric field is not constant for the coaxial case, and the highest electric field occurs at r=a.
Suppose the distance between b and a in the cylindrical case is the same as the distance between x2 and x1 in the planar, i.e., b−a=x2−x1=W. Now assume that highest value of the electric field in both cases just reaches the critical electric field EA such that
                                          E            A                    =                                                    V                O                cylindrical                                            a                ⁢                                                                  ⁢                                  ln                  ⁡                                      (                                          b                      /                      a                                        )                                                                        =                                          V                O                planar                            W                                      ,                            (        5        )            which, upon rearrangement, yields
                                          V            O            planar                                V            O            cylindrical                          =                              W                          a              ⁢                                                          ⁢                              ln                ⁡                                  (                                      a                    /                    b                                    )                                                              .                                    (        6        )            
If a<<b, then W=b−a≈b, so that the above result becomes
                                          V            O            planar                                V            O            cylindrical                          ≈                              b            /            a                                ln            ⁡                          (                              b                /                a                            )                                      >        1                            (        7        )            
Because a<<b, for similar chamber dimensions, it is seen that the voltage needed to reach EA for the planar device is always greater than that needed for the cylindrical device.
Atomic electrons elevated in energy through impact ionization can also generate additional free electrons. The excited atoms de-excite by the emission of ultraviolet (UV) light which, in turn, can remove loosely bound electrons from other atoms through the process known as photoionization. Such electrons from photoionization can cause problems. To understand this, let δ be the probability that a secondary electron produces a tertiary electron as a result of UV photoionization. If f is the gas multiplication from the initial avalanche, the overall multiplication from successive avalanches caused by the UV produced photoionization electrons is
                              M          =                                    f              +                              δ                ⁢                                                                  ⁢                                  f                  2                                            +                                                δ                  2                                ⁢                                  f                  3                                            +                              …                ⁢                                                                  ⁢                                  f                                    n                                                                                …                      ⁢                                                                                          ⁢                      1                                                                          =                                          ∑                                  i                  =                  1                                n                            ⁢                                                δ                                      i                    -                    1                                                  ⁢                                  f                  i                                                                    ,                            (        8        )            where i represents the consecutive avalanche waves (first, second, third, and so on) up to the final avalanche n. The quantity δƒ is strongly dependent upon the applied operating voltage. If δƒ<1 the series in Eq. 8 reduces to
                    M        =                              f                          1              -                              δ                ⁢                                                                  ⁢                f                                              .                                    (        9        )            
If, however, δf>1, the avalanching process becomes uncontrollable and the detector develops a self-sustaining discharge. This may occur when too high a voltage is applied (as in Region V of FIG. 1). Continuous waves of avalanches can occur if UV light released by the excited electrons ionize too many Ar atoms, and if the Ar atoms, when arriving at the cathode wall, strike with enough kinetic energy to cause the ejection of more electrons, as depicted in FIG. 5a. To prevent continuous waves of avalanching from occurring in the chamber after a radiation interaction, a quenching gas is added to the gas mixture, typically a polyorganic molecule. A common proportional counter gas is P-10, which is a mixture of 90% Ar and 10% methane (the quenching gas). When an ionizing particle enters the detector, it ionizes both the Ar and the quenching gas. However, as the Ar gas ions drift through the chamber, they transfer their charge to the quench gas molecules, which then continue to drift and carry the positive charge to the cathode wall. When a quench gas is struck by a UV photon or strikes the cathode wall, it dissociates by releasing a hydrogen atom rather than ejecting an electron, as shown in FIG. 5b. As a result, the quench gas prevents continuous waves of avalanches.
Multiwire Proportional Counter
Multiwire proportional counters, developed in 1968 by Charpak, are similar to single wire devices, except that they use a criss-cross array of wires. Typically there are two planar arrays of parallel cathode wires with the arrays positioned orthogonal to each other. One might consider one set of wires parallel to the x direction and the other set parallel to the y direction. In between the two cathode wire array planes is a parallel planar array of anode wires, which are typically arranged at a 45° angle to the cathode wires (see FIG. 6). As with the simple proportional counter, ionizing radiation produces primary electron-ion pairs in the detector gas. Electrons travel towards the nearest anode wires in the array, which then produce a Townsend avalanche of electron-ion pairs. The cloud of positive ions separate and travel towards the nearest cathode wires in the planes on both sides of the anodes. Hence, the position of the event is determined by which cathode wires deliver a signal on the x-y plane. Overall, the multiwire proportional counter can provide both energy information and position information of the ionizing event. Charpak was awarded the 1992 Nobel Prize in Physics for his invention of the multi-wire proportional chamber.
Neutron-Sensitive Proportional Counters
As with the ion chamber, proportional counters that are either coated with a strongly absorbing neutron reactive material or are filled with a neutron reactive gas can be used as neutron detectors. The most commonly used materials for proportional counter neutron detectors are the gases 3He and 10BF3, and the solid 10B. Although neutron sensitive, neither 10BF3 nor 3He are ideal proportional gases, but they perform adequately well. Because the device operates in proportional mode, a low resolution spectrum associated with the reaction product energies of the 10B(n,α)7Li reactions or the 3He(n,p)3H reactions can be identified, depending on the gas used in the counter. This prior art type of gas-filled neutron detector is depicted in FIGS. 8 and 9.
Shown in FIG. 8 is a prior art gas flow detector in which a neutron reactive gas 6 is constantly purged through the detector chamber composed of cathode walls 1 and lid 2. Voltage is applied to the electrodes 5, typically operated as the anodes. A neutron 8 enters the detector through a thin membrane 7 into the reactive gas 6 and is absorbed. The reaction results in the instantaneous emission of reaction products 9 which cause ionization 11 in the gas 6. The electrons are drawn towards the anodes 5, which cause a Townsend avalanche and voltage output pulse.
Shown in FIG. 9 is a prior art gas-filled detector in which a neutron reactive gas 6 is sealed in detector chamber composed of a cylindrical cathode 1. Voltage is applied to the electrode 5, typically operated as the anode. A neutron 8 enters the detector through the cathode wall 1 into the reactive gas 6 and is absorbed. The reaction results in the instantaneous emission of reaction products 9 which cause ionization 11 in the gas 6. The electrons are drawn towards the anode 5, which cause a Townsend avalanche and voltage output pulse.
The neutron detection efficiency can be increased by increasing the gas pressure of the counter, hence providing more neutron absorber. Typical pressures range from 1 atm to 10 atm. Electron and ion velocities decrease inversely proportional to gas pressure: consequently, increasing the gas pressure in the tube causes the counter dead time to increase. Gas-filled tubes come in a variety of sizes, ranging from small chambers only a few cm long and one cm in diameter to large chambers several feet long and several inches in diameter.
Unfortunately 3He is relatively rare gas that has become expensive in recent times, thereby driving up the cost of these gas-filled detectors. Further, 10BF3 is a poisonous gas and does have certain health risks associated with their production, use and disposal.
A better proportional gas such as P-10, a gas that is non-reactive with neutrons, may be used in the chamber if, instead of filling the chamber with a neutron reactive gas, the walls are coated with 10B. Unfortunately, the spectral features from such a device are harder to interpret due to interference from background gamma rays, and the total neutron detection efficiency is limited by the thinness of the optimum 10B absorber coating, typically only 2 to 3 microns thick.
Shown in FIG. 10 is a common design for a coated proportional counter used for neutron detection, in which a neutron reactive coating 12 is on the cathode wall 1. The detector is filled with a gas 13 generally not reactive with neutrons. Neutrons are absorbed in the reactive coating 12 which results in the emission of ionizing reaction products 9. Due to the geometry, and the fact that the reaction products are emitted in opposite directions, only one of the reaction products 9 can enter the detector gas. The result is a decreased amount of energy deposited in the detector than the total Q value of the reaction, resulting in less ionization 11. Further, due to self absorption of energy as the reaction product travels through the neutron absorbing film to the detector gas, more energy can be lost, a significant problem with this type of detector. Further, the total overall efficiency that can be achieved with the design is less than 10% detection of thermal neutrons.
Referring now to FIGS. 11 and 12, there is shown a prior art detector where metal washers 30 have been inserted down the axis of the cylindrical gas-filled detector. The washers 30 are separated by spacers 31 and the washers 30 are coated on both sides with a neutron reactive material 12. Although the design increases the overall efficiency of the detector, it has a limit to the efficiency that can be realized. Further, the detector of FIGS. 11 and 12 is designed to point, end to end, at the neutron source. Because of the geometry of the detector of FIGS. 11 and 12, neutrons will not be detected effectively if the detector is irradiated from the side, which is the preferred method of operating gas-filled neutron detectors. A practical device will be limited to less than 35% detection efficiency of thermal neutrons if the device is irradiated end on, reducing to almost 0% if irradiated from the side. As with the detector of FIG. 10, only one reactive product can enter the gas chamber because the other reaction product enters the metal washer 30.
Referring again to FIG. 8, there is illustrated a prior art gas-filled neutron detector, depicting a cross-section of a typical multi-anode gas-filled neutron detector. The detector is composed of a container 1 with a lid 2 that contains the neutron-reactive detector gas in the cavity 6. The detector gas is generally a material that reacts strongly with neutrons. Example neutron-reactive gases used in these detectors include 3He and 10BF3. A thin barrier 7 completes the detector enclosure. Aluminized Mylar is typically used as a thin barrier. The detector container 1 serves as an electrode. An additional electrode or series of electrodes 5 are provided, usually thin wires 5, in order to apply a voltage across the gas in the detector cavity 6. Neutrons 8 interact in the neutron reactive gas and subsequently cause the ejection of ionizing radiation 9. The ionizing radiation enters the gas and excites electron-ion pairs 11. These electron-ion pairs are separated by the applied detector voltage. Typically, a positive voltage is applied to the small wire electrodes 5, named the anodes. Electrons drift to the anodes, and upon reaching the anodes, create a Townsend avalanche, thus producing a much larger number of electron-ion pairs. The new positive ions drift toward the outer perimeter and the current produced by their motion is measured and recorded as a neutron interaction event. The neutron-reactive gas is constantly replenished thought ports 3 and 4. The detector efficiency of FIG. 8 can be high, often above 80%.
Referring again to FIG. 9, there is illustrated a prior art gas-filled neutron detector, depicting a cross-section of a typical coaxial single anode gas-filled neutron detector. The detector is composed of a container 1 that contains the neutron-reactive detector gas in the cavity 6. The detector gas is generally a material that reacts strongly with neutrons. Example neutron-reactive gases used in these detectors include 3He and 10BF3. The detector container 1 serves as an electrode. An additional electrode 5 is provided, usually a thin wire 5, in order to apply a voltage across the gas in the detector cavity 6. Neutrons 8 interact in the neutron reactive gas and subsequently cause the ejection of ionizing radiation 9. The ionizing radiation enters the gas and excites electron-ion pairs 11. These electron-ion pairs are separated by the applied detector voltage. Typically, a positive voltage is applied to the small wire electrode 5, named the anode. Electrons drift to the anode, and upon reaching the anode, create a Townsend avalanche, thus producing a much larger number of electron-ion pairs. The new positive ions drift toward the outer perimeter and the current produced by their motion is measured and recorded as a neutron interaction event. The detector efficiency of FIG. 9 can be high, often above 80%. The gas in this style of detector is not replenished and can be exhausted over a period of time.
Referring again to FIG. 10, there is illustrated a prior art coated gas-filled neutron detector, depicting a cross-section of a typical coaxial single anode gas-filled neutron detector. The detector is composed of a container 1 that contains the non-reactive detector gas in the cavity 13. The detector gas is generally a material that does not react strongly with neutrons. Example neutron-reactive gases used in these detectors include Ar and P-10, a mixture of 10% methane and 90% Ar. The detector container 1 serves as an electrode. An additional electrode 5 is provided, usually a thin wire 5, in order to apply a voltage across the non-reactive gas in the detector cavity 13. Neutrons 8 interact in the neutron reactive coating 12 and subsequently cause the ejection of ionizing radiation 9. The ionizing radiation enters the gas and excites electron-ion pairs 11. Note that the configuration does not allow for both reaction products 9 to enter the detector cavity 1, but instead only one reaction product 9 can enter the cavity 1. These electron-ion pairs are separated by the applied detector voltage. Typically, a positive voltage is applied to the small wire electrode 5, named the anode. Electrons drift to the anode, and upon reaching the anode, create a Townsend avalanche, thus producing a much larger number of electron-ion pairs. The new positive ions drift toward the outer perimeter and the current produced by their motion is measured and recorded as a neutron interaction event. The detector efficiency of FIG. 10 is usually low, limited to less than 10%.
Referring again to FIG. 11, there is illustrated a prior art coated gas-filled neutron detector, depicting a coaxial single anode gas-filled neutron detector. The detector is composed of a container 1 that contains the non-reactive detector gas in the cavity 13. The detector gas is generally a material that does not react strongly with neutrons. Example neutron-reactive gases used in these detectors include Ar and P-10, a mixture of 10% methane and 90% Ar. The detector container 1 serves as an electrode. An additional electrode 5 is provided, usually a thin wire 5, in order to apply a voltage across the non-reactive gas in the detector cavity 13. Torus shaped metal washers 30 are coated with neutron reactive material 12. Neutrons 8 interact in the neutron reactive coating 12 and subsequently cause the ejection of ionizing radiation 9. The ionizing radiation enters the gas and excites electron-ion pairs 11. Note that the configuration does not allow for both reaction products 9 to enter the detector cavity 1, but instead only one reaction product 9 can enter the cavity 1. These electron-ion pairs are separated by the applied detector voltage. Typically, a positive voltage is applied to the small wire electrode 5, named the anode. Electrons drift to the anode, and upon reaching the anode, create a Townsend avalanche, thus producing a much larger number of electron-ion pairs. The new positive ions drift toward the outer perimeter and the current produced by their motion is measured and recorded as a neutron interaction event.
Referring again to FIG. 12, there is illustrated a prior art coated gas-filled neutron detector, depicting a coaxial single anode gas-filled neutron detector. The detector is composed of a container 1 that contains the non-reactive detector gas in the cavity 13. The detector gas is generally a material that does not react strongly with neutrons. Example neutron-reactive gases used in these detectors include Ar and P-10, a mixture of 10% methane and 90% Ar. The detector container 1 serves as an electrode. An additional electrode 5 is provided, usually a thin wire 5, in order to apply a voltage across the non-reactive gas in the detector cavity 13. Torus shaped metal washers 30 are coated with neutron reactive material 12. The metal washers 30 are separated by insulting spacers 31. Neutrons 8 interact in the neutron reactive coating 12 and subsequently cause the ejection of ionizing radiation 9. The ionizing radiation enters the gas and excites electron-ion pairs 11. Note that the configuration does not allow for both reaction products 9 to enter the detector cavity 1, but instead only one reaction product 9 can enter the cavity 1. These electron-ion pairs are separated by the applied detector voltage. Typically, a positive voltage is applied to the small wire electrode 5, named the anode. Electrons drift to the anode, and upon reaching the anode, create a Townsend avalanche, thus producing a much larger number of electron-ion pairs. The new positive ions drift toward the outer perimeter and the current produced by their motion is measured and recorded as a neutron interaction event.
Geiger-Mueller Counters
Although Hans Geiger originally created the gas-filled detector in 1908 (with Ernest Rutherford), the device used today is based on an improved version that his first PhD student, Walther Mueller, constructed in 1928. Hence, the proper name for the device is the “Geiger-Mueller” counter. The original “Geiger” counter was sensitive to alpha particles, but not so much to other forms of ionizing radiation. Mueller's improvements included the implementation of vacuum tube technology, which allowed for the device to be formed into a compact and portable tube sensitive to alpha, beta and gamma radiation. In 1947, Sidney Liebson further improved the device by substituting a halogen as the quenching gas, which allowed the detector to operate at lower applied voltages while lasting a significantly longer time. Geiger counters are typically arranged in a coaxial configuration, in which a thin anode wire is projected inside a tube that serves as the cathode. A high voltage is applied to the central anode wire, while the cathode is held at ground, as shown in FIG. 2.
Geiger-Mueller counters are operated in Region IV of the gas counter curve. The device depends upon gas multiplication as a signal amplification mechanism, much like the proportional counter, however a single important difference is that, at any specific applied voltage, all output pulses from a Geiger-Mueller counter are of the same magnitude regardless of the ionizing radiation energy or type. Hence, Geiger-Mueller counters do not intrinsically possess the ability to discern between alpha, beta, or gamma radiation, nor can they distinguish between different energies of these radiations.
When an ionizing particle enters a Geiger-Mueller counter, the counting gas becomes ionized creating a small cloud of electron-ion pairs (depicted in FIG. 7(a)). Because a high voltage is applied to the anode, the device operates in region IV of the gas curve. The electrons drift rapidly to the anode while the ions slowly drift towards the cathode, as shown in FIG. 7(b). When the electrons enter into the high electric field near the anode above the critical field EA needed to produce avalanche ionization, they gain enough kinetic energy to produce more electron-ion pairs through impact ionization, and a large and dense cloud of electron-ion pairs is formed. In addition, impact ionizations excite electrons in some gas atoms which emit UV photons when they de-excite and produce more ionization through photoionization. This large accumulation of positive ions near the anode affects the electric field and reduces its strength. These processes are depicted in FIGS. 7(c) and (d). There is a point at which the large accumulation of space charge around the anode increases so much that the electric field is reduced below the critical field strength EA needed to sustain avalanching; hence, impact ionization ceases, as shown in FIG. 7(e). The positive ions drift to the cathode, which produces the output pulse for the detector. As they move towards the cathode, the electric field near the anode recovers to full strength once again, and the detector is now set to detect the next radiation interaction event, as depicted by FIG. 7(f).
A few matters should be noted: (a) the electric field in the detector increases with an increase in applied voltage; (b) the Geiger-Mueller discharge ceases when the electric field is reduced below EA at the anode and, therefore, the positive ion accumulation density must increase with applied operating voltage to stop the avalanche; (c) to prevent more electrons from being ejected when the ions strike the cathode, a quenching gas must be used just as with the proportional counter; and (d) the entire Geiger discharge process is slower than that of a proportional counter, mainly because of the time required to produce the dense cloud of positive ions. Hence, the size of the output pulse is determined by how much space charge must accumulate to reduce the electric field below EA and not the energy deposited within the detector. As a result, the pulse height for various energies of α-particles, β-particles, and γ-rays are all the same, within statistical variation, and the output pulse height is predetermined by the applied operating voltage. Dead times for Geiger-Mueller counters can be on the order of 10 times longer than those of proportional counters of similar size. Lastly, because Geiger-Mueller counters are typically closed tubes, the quenching gas inside can be exhausted over time if traditional organic molecules such as the methane component of P-10 gas are used. Instead, Geiger-Mueller counters use halogens for a quenching gas, in which the diatomic molecules dissociate when they strike the cathode. Halogens, unlike methane, can heal themselves by recombining into diatomic molecules, thereby extending the life of the gas in the detector.
Considering equations 1 and 2, gas-filled detectors relying upon neutron reactive coatings are limited to low efficiencies due to reaction product self-absorption. Neutrons impinging upon a cylindrical gas-filled detector with a coating on the inner tube surface must first cross the coating before entering into the detector gas, and those not absorbed must again cross the coating while exiting the device. As a result, the maximum efficiency that the device can have will be limited to less than 10% thermal neutron detection efficiency. Detectors with washers coated with neutron reactive material aligned down the axis of a detector can increase the neutron detection efficiency, but are ineffective when irradiated from the side and are designed to point end on at the neutron source.